1. Field of the Invention
The present invention is directed to modification of metal oxide particles by injection into a flame of moderate temperature by liquid feed flame spray techniques.
2. Background Art
Particles have been supplied into high temperature plasma or flames, i.e. above 2000° C., to melt or even vaporize the particles for use in preparing coatings. However, the art is silent regarding lower temperature processes whereby phase may be altered, for example a crystalline phase or an amorphous phase, or both, or whereby hollow or core/shell particle morphology may be created from particles in the micron range, and preferably those of nanometer size, while retaining their particulate nature.
There are numerous methods of preparing ultrafine, and nanosized oxide powders using chemical compounds as precursors. These methods are summarized in detail in “Mixed-metal Oxide Particles by Liquid Feed-flame Spray Pyrolysis of Oxide Precursors in Oxygenated Solvents,” A. C. Sutorik, R. M. Laine, J. Marchal, T. Johns, T. Hinklin, WO 03/070640 A1 published Aug. 28, 2003, which is incorporated herein.
The literature also describes the injection of preformed particles, or occasionally their precursors, into a plasma or related high temperature flames, typically >2000° C., such that the particles are vaporized or fully melted to subsequently produce coatings such as thermal barrier coatings. In these instances, the particles are 10-50 μm diameter which are, in some instances, formed from nanosized particles in the range of 6-70 nm. The larger particles or precursors are typically injected off-axis into a plasma arc, and generally result in relatively smooth coatings, as the molten particles “splat” onto the substrate. A similar process has been used for many years to produce boules of alumina (sapphire, ruby).
Solution injection of precursors may also be used to prepare particles and/or coatings. In such processes, rapid evaporation of solvent initially forms more concentrated admixtures which then, under the high temperature pyrolysis conditions, further undergo such processes as breakup, precipitation, and/or gelation, followed by pyrolysis, sintering, and if the flame is hot enough, fusion. Often, the fused particles are much larger than the particles initially formed, as the result of forming agglomerates prior to fusion. The process is also useful for forming thermal barrier coatings. However, their particulate nature is substantially lost. Alumina, alumina/zirconia and also alumina/titania mixtures are used commonly for the formation of such thermal barrier coatings. In most instances, the alumina formed is the gamma phase with small amounts of alpha. For example, Trice et al., “Deformation Mechanisms in Compression-Loaded Stand-Alone Plasma-Sprayed Alumina Coatings,” J. AM. CERAM. SOC. 83, p. 3057-64 (2000), obtain alumina systems wherein the alpha content is ≈10%.
It would be desirable to provide a process in which particles are obtained with morphologies and other characteristics which cannot be obtained by in situ formation of particles from liquid or gas phase precursors. It would be further desirable to uncouple the chemical makeup of such particles from that of their precursors. It would be further desirable to enable substrates to be coated with coatings wherein the particulate nature of the particles used in preparing the coating is retained.
Thus, in one aspect, the invention pertains to injection of nano-, ultrafine- and micron-sized ceramic oxide particles into low temperature (400-2000° C.) oxidizing, reducing or neutral flames individually, in combination with each other, or coinjected coincidentally or serially downstream, optionally also with chemical precursors, and as a result, their phases, particle sizes and size distributions, phase compositions and physical and chemical properties significantly modified by exposure to the flame and to the coinjected or serially injected materials. During the process, novel and unexpected particle morphologies and size distributions can be generated through the use of combinations of particles, or combinations of particles and precursors coinjected, or serially injected downstream from the initial liquid feed flame spray combustion unit. This process includes the coating of supports and substrates with combinations of metal oxides and metals for development of new catalysts, photonic materials, sensing materials, thermal barrier coatings, abrasion and corrosion resistant coatings, prosthetic ceramic materials, conducting materials, transparent ceramics, etc. In preferred aspects, the products may be α-alumina or combinations of α-alumina coated with other metal oxides or products of the reaction of γ- or α-alumina nanoparticles with coinjected or serially injected components including other metal oxide powders or precursors or combinations thereof; are large clay particles transformed to dense spherical or hollow spherical particles that can be coated with alumina or related hard ceramic nanopowders for applications ranging from novel abrasives to insulating packing materials for construction materials; metal oxide coatings on alumina, yttria, YAG, or titania, NiO or other transition, lanthanum or actinide or main group metal oxide such that the coatings can be modified to become catalytically active or electrically conducting, and may be transparent as well, for applications ranging from self-cleaning photocatalytic materials to fuel and fine chemical processing catalysts to antistatic and/or conducting coatings; and products wherein a glass, ceramic, metal or mixed phase material is placed such that the hot particles impinge on the heated surface and adhere to form coatings that are corrosion resistant, abrasion resistant, catalytically active, thermally robust, insulating, conducting or combinations thereof; while the particles remain substantially in particulate form, and are preferably either dense, porous, or exhibit a gradient from porous to dense or vice versa.